text:
What additional considerations do pilots have when deviating from the planned routing on flights dispatched under Method 2?
This article about Method 1 and Method 2 gives a brief explanation of Methods 1 and 2 in terms of dispatching flights under 14 CFR 121. Here's an excerpt.

FAR 121.191 requires that we dispatch our aircraft on a routing, weight and expected enroute temperature, that complies with a minimum terrain clearance requirement after an engine failure.
It also requires that we have a plan in place that allows a diversion to an alternate airport enroute,  following an engine failure and drift down, with a specific terrain clearance if we do not have the performance to continue to the destination.

My question about flights dispatched under Method 2 concerns route changes. In practice, I'm quite sure that pilots operating aircraft with a Method 2 dispatch release deviate from the planned route on the dispatch all the time. Reasons might include a shortcut request to air traffic control or deviations for weather. Weather deviations seem like the most relevant here because they can sometimes be quite significant.
What additional considerations do pilots have when accepting route changes on flights dispatched under Method 2? Couldn't changing the route render the Method 2 calculations irrelevant?

I think the title needs to be slightly modified... "Method 2" is not really an industry term, but instead something specific to this page's explanation or possibly Jeppeson's flight planning software and/or JetBlue's GOM.  In a more generic sense, pilots might deviate from planned routes where terrain clearance was taken into account in the release.  Perhaps: "What additional considerations do pilots have when deviating from the planned routing on flights dispatched with terrain clearance requirements?" (or something like that)

"Method 1" and "Method 2" are common enough that I'd rather make the question more specific than lose that detail. It's a matter of opinion whether or not they are "industry terms." I believe they are from my conversations with pilots from other US 121 pilots.

As a Former Dispatch Examiner and an Airline Part 121 pilot for most of my career I think what you are looking for is this:
Unless the terrain is such that they must adhere to or close to the desired routing and they are notified of this, most deviations are inconsequential. It also depends on how many engines are on the aircraft. A twin engine aircraft must land at the nearest suitable airport in the point of time after an engine failure. Although it is wise to land with a greater than 2 engine aircraft, that is not the requirement. If an engine fails after takeoff before reaching cruise altitude the PIC should consider terrain clearance in his decision to continue, deviate or return. The PIC is responsible for his decisions and can deviate or not deviate if he feels his decision is as safe as the alternative. After drift down, most jets should clear all terrain with few exceptions but he should always consider obstacle clearance.
Remember everything in preflight must meet regulatory criteria. Once airborne the real world situation can alter and often does alter planning and some regulatory requirements. A lot of planning regulations go away after takeoff. The PIC is the final authority in all decisions after departure even though you both are considered responsible and should agree. If the crew has time to coordinate with Dispatch, then it is the dispatcher's job to warn the crew of all known hazards they may have overlooked while dealing with the in-flight problem even if they were briefed prior to takeoff.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Why do (jet) airplanes fly at tropopause (and not higher)
I've got a question which is nearly answered by the following thread however not fully to my satisfaction.
Why do jet engines get better fuel efficiency at high altitudes?
The question ultimately comes down to:
Why do jet airplanes fly around tropopause and not higher ?
In the following I will relate what I believe to know and try to explain the reasoning in my mind. Please correct me in all cases where my reasoning is not entirely correct.
Here is what I get.
-Fuel efficiency becomes better with altitude
This is because even though air gets less dense in altitude (amount of oxygen per volume decreases) the ram effect compensates so that the same amount of air enters the compressor. So in comparison with low altitude,  in high altitude the same amount of air is entering the compressor so the same amount of air (number of oxygen particles) enters the combustion chamber. If the same amount of air enters the combustion chamber the same amount of fuel will be used to combust. so the same pressure and temperature is created inside the combustion chamber.
What we need for higher propulsion of the gas is the highest possible pressure difference in comparison to the ambient pressure. Higher pressure in the combustion chamber is limited by the maximum temperature the turbine blades are able to resist, so the only way to get more propulsion is a lower ambient pressure wich is the case in higher altitude.
On top of that, thermal efficiency increases with lower temperatures. meaning more of all the energy inside the fuel is transferred into heat and pressure  when combusted. Only this is no more than approximately 5% in higher altitude.
These two lead to my conclusion that in altitude you will need high speed (to create ram) this will make your engine give more propulsion/thrust for the same amount of fuel burned when flying at the same speed at lower altitude. So you are able to reduce fuel burned to create the same thrust in high altitude and thus increasing fuel efficiency.
What I also know is that airplanes try to fly around the tropopause. Does it have anything to do with the characteristics of the tropopause ?
because  why don't they fly higher ? is this because temperature doesn't decrease anymore ? (this is the original story I recall from my classes a few years ago but I can't find the reasoning behind it any more) I find it now hard to believe since thermal efficiency is almost negligible.
Perhaps this is because the structure of the airplane is not able to withstand the pressure difference due to the fact the cabin pressure needs to remain below 10.000 feet ? In this it would still be true that an airplane flying higher and higher than the tropopause would get more and more fuel efficient. But also this I find hard to believe because with newer and modern composites an aeroplane should be able to withstand higher pressure differentials without adding weight and so be able to increase fuel efficiency by going higher. Cheaper and easier than developing more efficient engines I believe.
So where am I wrong ?

@Federico this question seems more centered on "why isn't getting above the tropopause worth the fuel efficiency gains?"

@BSteinhurst to me the answer is contained in the answers to that question. we'll see also what the OP thinks and the rest of the community.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Does it matter in which country I obtain my EASA Part 66 AME licence?
I have completed EASA Part 66 B1.1 exam modules (some from the UK CAA and some from the German 147 organisation).
My queries are:

Can I apply to any member state (e.g., Belgium, The Netherlands, Germany, Ireland, UK, etc.) to get my initial Part 66 licence?
I have already emailed few country’s licensing departments; they declined and advised me to apply only to the UK CAA.

If I can apply to any member state, why did I get these replies? If I can’t, why not?

Does it affect job prospects in the UK if someone has a Part 66 license issued by another European country? (And vice versa?)

To which authority (country) can I apply to get my Part 66 licence as soon as possible if I am eligible to do so.

Does it affect aircraft type rating endorsement in future on a Part 66 licence? For example, if my license is issued from Belgium but I’m working in the UK.

When you have to deal with the authority, the theory and the practice varies a lot. So, there are things that theoretically you should be easily able to do, but that the authority won't accept, for no real reason. Sometimes it's just how the guy at the desk woke up in the morning.
Anyway:
1) Yes, you can apply in any country of the EU28. I know Switzerland won't accept your appliances unless you work in Switzerland, don't know about Norway and Iceland. Have no idea regarding the new situation in UK after the Brexit.
2) Because Europe is not a federation of States like in USA, and each authority want to impose own power and rules and protect our market. Ask them why.
3) Theoretically no. Pratically, maybe.
4) All EASA countries. Where do you work? Where did you get your theoretical modules and practical experience? Try starting from there.
5) Repeat the question again, it's not very clear to me.
To apply you need to compile and send the EASA Form 19 (You can find it here) to any European competent authority.
You can also find the Annex III, the Part 66 of the Easa Regulation 2042/2003 here, with all the pertinent information. (In particular check the 66.A.10 Application)

Do you have any references? Preferably to actual regulations.

@JanHudec I updated the answer. Is it clear like this?


text:
Are there any incidents caused by rudder failure in Military jets?
Based on this question about the rudder position of the F/A-18 Fighter Jet.
Quote from the user Rhino Driver:

The logic has its limits though and if the pilot exceeds 14 AoA in a landing mode (flaps half or full), the FCS may accidently depart the jet. This is obviously bad 200 ft above the ground.
As stated before, there are times when the rudders may suddenly fare themselves which can have devastating effects during the final portion to land. Rudder failures are serious problems and should be treated as such.

Are there any known incidents where the pilot (as example) exceeded 14 AoA in landing mode and the FCS accidently departed the jet?
The FCS has to play a certain role in the incident (like in the example) or like the incident No. 3 from the user aeroalias.

There are certainly a large number of incidents/accidents in military jets involving rudders. I've listed a few cases, not necessarily in takeoff/landing.

A Block 25 F-16C suffered structural failure during flutter tests with the AMRAAM missile. The rudder delaminated and tore off, and the pilot managed to land the aircraft.

Source: f-16.net

B-52 vertical stabilizers were prone to failure during operation at low altitudes. While under testing, a B-52H suffered catastrophic failure of vertical stabilizer. The pilots managed to land the aircraft with help from Boeing test engineers.

"Boeing B-52 with no vertical stabilizer" by United States Air Force - USAF. Licensed under Public Domain via Commons.
However, in other cases, the same failure caused fatal crashes.

A USAF investigation blamed rudder failure for the loss of an F-15 near Hawaii in 2008. The investigation concluded that most probably both rudders in the jet failed due to failure of the aileron-rudder interconnect.
F-14 suffered from rudder hardover incidents, one of which lead to loss of the aircraft. Investigations found the reason was due to mechanical failure of a stop plate attached to the hydraulic actuator.

I didn't know there were so many. I'm going to edit my question to be more specific what I want to know.


text:
Cruiser-Feeder Concept - feasible?
The people from cruiser-feeder started a project called RECREATE.

However, for the concept with fuel transfer from feeder to cruiser (civil air-to-air refuelling operations), the results of our collaborative research indicate a fuel burn reduction potential on isolated aircraft level between 11% and 23 % for a typical 6000 nautical miles flight with a payload of 250 passengers.

That sounds very good, doesn't it? I answered this question about civil air-to-air refuelling with the cruiser feeder concept. A User said:

with the Breguet equation it is easy to see that the savings are modest.

And:

This concept will never be certified

They even created a high-realistic simulation environment in which they tested the refuelling-process with real pilots as you can see in this video.
The feedback from the pilot is very good and promising that the concept might become real.
I'm curious why the savings are modest, because the planes can reduce the fuel burn from 11% to 23% and why this concept will never be certified.
Can the cruiser-feeder concept become real? That a feeder pumps fuel to a cruiser after he took of?
When does the use of the feeder burn more fuel than it saves?
What could stand in the way to make the RECREATE Project become real?

the video as well as the website are just marketing tools to hopefully get venture capitalists to give them money which will then go who knows where, but most likely into the pockets of the company owners right before they jump ship.

@jwenting Can you provice any source on that assumption? Isn't that assumption a bit extreme? I mean the project is founded by the EU's Seventh Framework Programme for research, technological development and demonstration...

The EU funding should be the red herring here. Sorry to be blunt, but EU bureaucrats are not exactly known for their prescient use of public money. The numbers simply don't add up. Please read and understand what I have answered here - those 23% are totally incredible.

The system is highly impracticable for a number of reasons.

The referred paper acknowledges that the fuel savings due to existing tankers are negligible and a new design is required. I doubt anyone is going to develop this. Even the military uses modified civil airliners for its tankers needs. Development of the tanker as outlined in the paper will cost billions of dollars and no one is going to do that for a handful of tankers.
Even if such a tanker is built, the existing aircraft will require significant modifications for in flight refueling (assuming that the tanker is standardized from day one, which is practically not possible; even the US armed forces use two different type of tankers). Basically, the structural weight goes up,which reduces performance and increases fuel consumption.
Then the pilots have to be trained for operations of both the aircraft. In flight refuelling is potentially dangerous.See http://foxtrotalpha.jalopnik.com/aerial-refueling-hooking-up-is-hard-to-do-1559495801. Nobody's going to take that risk with civil operations.
The study is extremely simplistic. modern civil aircraft fly in a number of 'flight levels', separated by set altitude. So, for refueling, either the tanker or receiver should climb or descend the difference in altitude, and that burns up more fuel.
If carrying extra fuel in the aircraft is dead weight, it makes no sense to carry it in another aircraft and pay for its operations and maintenance.
It is not correct to compare the military and civil operations. Their priorities, methods and end goals are different. What looks good on paper may not work on real life. Remember Concorde?

The savings for the cruiser/feeder concept come from two main areas:

Cruiser fuel saving as it does not have to carry the fuel for the second leg of the journey during the first leg (first leg is before refuelling, second leg is after). The feeder is much smaller and flies a shorter distance than the cruiser, this is what makes the process efficient.
Smaller cruiser as you do not need as large an aircraft to fly the same distance. E.g. rather than a 6000nm cruiser you can design an aircraft for a design range of 3000nm. It there has a much smaller operating empty weigh etc. and is therefore more fuel efficient.

So the answers talking about using simple calculations such as the Breguet equation are misguided, as they have completely ignored one of the main areas of fuel saving, the fact that you use a smaller and more fuel efficient aircraft to fly the same distance.
The main issue is that you can largely get the same effect with staging the journey. Rather than having a feeder aircraft, you simply land and refuel. This obviously eliminates the feeder, but has its own problems (longer journey times, more take off/landings at busy airports).
There are clear fuel savings to be made with the cruiser/feeder concept for a specific subset of journeys. It just seems a large investment when the same effect can be implemented with staging much more easily.

how would you save fuel by landing and taking off again when this operations are the most expensive under the fuel consumption point of view?

The aircraft is not carrying the fuel for the second leg of the journey during the first leg. Over long distances e.g. 6000nm this fuel weight is substantial.  You also have a much smaller aircraft i.e. rather than having an aircraft that is designed for 6000nm you design an aircraft for 3000nm, it weighs much less and is more fuel efficient. The extra fuel burnt in taking off and landing one more time is a lot smaller compared to the saving you would see when staging a long flight.

I am unconvinced. Do you have any source or computation done? Or is it speculation done by you?

ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070032063.pdf

But if the cruiser doesn't take off and land then the feeder has to. Either way you can't avoid a takeoff/ landing process. Sounds dubious to me.


text:
Was the intended purpose of the F-117 Nighthawk originally a stealth-fighter?
Based on this question, I learned that the F and A-Letters in the naming of (military) aircraft reflects what their intended purpose is.
A: Attack (Air-to-ground)
F: Fighter (Air-to-air)

their names mostly reflect what their intended purpose is rather than their actual purpose.
It is to be noted that the naming convention is followed only loosely in some cases. A good example is the F-117 Nighthawk, which is not a fighter aircraft in any sense of the word.

from aeroalias answer from this question.
The F117 isn't capable of attacking Air-Targets, it's only used as an attack aircraft. But the name of the Nighthawk is F-117 (Fighter).
Question:

Was the idea behind the Nighthawk originally a stealth-fighter
(Air-to-Air) aircraft but was later changed to a stealth-attack
aircraft? That would explain the name 'F-117'.
Are there other attack-aircraft with the F in the name?
Are there fighter-aircraft with an A in the name?

The F/A/B etc designations haven't always been strictly adhered to. For example the F-35 (and to a lesser extent F-22) should both really be F/A as they have ground attack capability. Among others. And the A-10 has some air to air ability. That said, I believe the F-117 is the only "Fighter" designated aircraft to have no air to air ability

According to Wikipedia: "A televised documentary quoted a senior member of the F-117A development team as saying that the top-notch USAF fighter pilots required to fly the new aircraft were more easily attracted to an aircraft with an "F" designation for fighter, as opposed to a bomber ("B") or attack ("A") designation"

F/A-18 is one of those aircraft that are a fighter and can attack ground targets.

@ethan that's why it's called Fighter / Attack - 18, In my question 2 & 3 I'm looking for an attack-aircraft only able to attack air-to-air targets (no air-to-ground) or an fighter aircraft only able to attack air-to-ground targets (like the F-117).

The F-18 Hornet is a fighter only. They added ground attack capabilities to the F/A-18 Super Hornet. The F-117 is a continuation of the pre-1962 black ops numbering system starting with F-110 (skipping F-111 because that was already being used). Several of the F-XXX series were captured Soviet aircraft.

@TomMcW First, there has never been an "F-18" in active US service. Before the Hornet ever entered service, its fighter and attack variants were merged, and the very first active ones had ground attack capability. The difference with the Super Hornet is not the addition of attack capability.  Second, F-XXX was not a continuation of a special black ops scheme; pre-1962, the USAF was actually up to the 100s, and it only became black-ops after 1962.

@cpast I think that's basically what I was saying about the numbering system, just worded it oddly. The point is F-117A is a black ops/ classified designation that was kept after it was made public. I thought I read somewhere that when the F-18 and A-18 variants were merged they changed the name to Super Hornet, but I prob read it wrong. (where's RhinoDriver when you need him?) So what is the diff with the Super Hornet?

@TomMcW I think that would make a own question. The main-difference between the Hornet / Superhornet are the size / thrust, avionics, sensors, a more modern cockpit, friend-to-friend refuelling capacities and so on...

The short of it is that the "A" designation for a ground-attack aircraft has historically been a Navy distinction. The USAF/USAAF/USAAC has not traditionally differentiated between an aircraft designed for tactical ground strikes versus a dogfighter/interceptor. Most WWII-era Air Force planes were capable of both, and would be called "multirole fighters" in today's parlance (including the famous P-38, P-40, P-47 and P-51). Between WWII and the adoption of the 1962 Tri-Service Aircraft Designation System, the USAF used "F" almost exclusively for any one- or two-man aircraft performing either the air-to-air or tactical air-to-ground role (using "B" for aircraft intended for larger-scale strategic bombing). Thus, the "Century Series" aircraft, many of which were primary ground attack aircraft, all got the "Fighter" designation even when their mission profile was exclusively air-to-ground, such as the F-105 and F-111.
The Tri-Service Aircraft Designation System was patterned most closely to the Air Force's pre-existing system, but resetting all the counters (the Navy's system involved much lower numbers, many of which were grandfathered into the new system) and adding mission designations common in the other branches, including the Navy's Attack designation. The Air Force has more or less ignored many of the finer points of this system when designating its own aircraft; the A-10, and its A-X competitor the YA-9, are the sole airframes designed primarily for the USAF to carry the Attack designation (which is fitting as both planes were designed from the ground up to kill tanks and other armor). The other A-designated aircraft to enter service with the Air Force were Navy planes first (including the A-1 Skyraider and A-7 Corsair II).
The F-15E should, by tri-service rules, be designated AF-15E (emphasizing its modified mission of tactical strikes over its air-superiority pedigree). "F/A" as used for the Hornet is also non-compliant, and stems from an original desire to have separate fighter and attack variants of the Hornet which ended up being rolled into one plane. "FA" or "AF" would suffice not only for the Hornet, but also the F-16 Falcon and the F-35 Lightning II (yet another aberration, this time in the numbering, keeping the number from the X-35 tech demonstrator instead of being the F-24 as expected).
The F-117 is a very special case. The airframe received its designation after 1962 and so should have been designated either A-14 (as its mission has always been tactical ground strikes) or F-19 (keeping with the USAF's tendency to label any light offensive aircraft a "fighter"). Development of the F-117 from the Have Blue project was an open secret, and it is widely believed that the Nighthawk is the missing "F-19" in the U.S. numbering system (the F-20, a variant of the F-5 powered by a single larger turbofan, having been announced and demonstrated long before the F-117's existence was officially acknowledged), with the pre-1962 designation intended as misinformation.

Re "The short of it is that the "A" designation for a ground-attack aircraft has historically been a Navy distinction. The USAF/USAAF/USAAC has not traditionally differentiated between an aircraft designed for tactical ground strikes versus a dogfighter/interceptor. "-- I don't think this is entirely true.  I can't think of any WW2 (or earlier) USAAF/USAAC aircraft bearing a "P" designation that was originally designed for ground attack. For example, the original ground attack version of the P-51 was called the A-36, not the P-xx or the P-51x.

I can't think of any WW2 (or earlier) USAAF/USAAC aircraft bearing a "P" designation that was originally designed exclusively or primarily for ground attack. For example, the original ground attack version of the P-51 was called the A-36, not the P-xx or the P-51x.  Sure, in the Korean war,  P-51's (by then called F-51's) got repurposed for an exclusively ground-attack role,  but that's another story.

@quietflyer prior to the utilisation of P series aircraft in ground attack roles, policy was to only use light bombers in that role. Aircraft like the B-23 (a failure) and B-29 were intended for this role. Hence the lack of dedicated ground attack aircraft with a P designation.

@jwenting -- "ground attack" is usually used to mean something more narrow than just any sort of dropping bombs; neither the B-23 nor the B-29 would normally be considered either "ground attack" aircraft or "light bombers".  There was a whole series of single and twin-engine aircraft w/ "A" designations and most of these would be called "light bombers" .  Douglas A13/A17, A-20 Havoc, A-36 Apache, A-26 Invader (later renamed B-26 after retirement of B-26 Marauder) are the only ones that pop into my mind right now but there were more, some of which were pre-WW2 "duds" that saw no wartime usage.

Comment above is specifically about WW2 and earlier, of course. Also, second - to - last sentence in very first comment should have read "designed PRIMARILY for ground attack". Also, note that most of the famous fighter- bombers of WW2 were originally envisioned as pure fighters.

@quietflyer meant B-26, obviously. Both the B-23 and B-26 were intended for the light/medium bomber role which were the aircraft slated for support of ground troops.

@jwenting -- the fact remains that there was a USAAF/USAAC "A" designation long predating WW2.  Obviously this is fertile ground for hair-splitting but in general "B" was used for bombers considered to be "medium" or "heavy" and "A" was used for bombers considered to be "light".   That's why there was simultaneously a B-26 and A-26, referencing two entirely different aircraft.  See previous comments fo other specific examples.

It may be the case that the post-WW2 USAF saw the "A" designation as superfluous for a good number of years, but the claims in the answer above go far beyond that and are not entirely warranted.

The answer could be improved by dropping the first and third sentences, and dropping "/USAAF/USAAC" from the second sentence (or entirely dropping the second sentence as well.) The third sentence in particular is not inaccurate but also is not particularly relevant, since the "A" designation was in fact in use by the USAAF throughout WW2, and all the P-designated aircraft mentioned in the sentence were originally envisioned exclusively or primarily as air-to-air fighters.

("fo" was typo; should be "for")

The 1948 re-designation  of the A-26 Invader as "B-26" is interesting and supports both the point made in this answer that for some time interval after WW2, the newly-created USAF viewed the "A" designation as unnecessary, and the point that the "A" designation was in use by the USAAC/USAAF throughout WW2 as well as much earlier.

a. No. The F-117 Nighthawk was intended to be a stealth attack aircraft from day zero. At no point was the Have Blue program,  which led to F-117, intended to produce a fighter.

"DARPA USAirForce HaveBlue" by US Air Force Photo. Licensed under Public Domain via Commons.
It came to be known as the F-117 as a result of a number of unrelated reasons. The aircraft was assigned a radio call sign '117' during testing in the range of the Soviet aircraft being tested in US and the number stuck.
Also, the 'F' in the designation appears to have been allowed by the USAF in order to confuse the Soviet intelligence. Technically, the aircraft should've had the number F-19, which was never used.
b. Another very good example of an attack aircraft having the 'F' moniker is the F-111 Aardvark, which was developed in the same time period.

"F-111F dropping high-drag bombs" by Service Depicted: Air ForceCommand Shown: F3253Camera Operator: STAFF SGT. DAVID S. NOLAN - ID:DFST8711197. Licensed under Public Domain via Commons.
Perhaps more confusingly, after the F-111 was retired, its duties were taken up by the F-15E strike Eagle, among others.
c. Most of the current aircraft are multirole and are capable of carrying out the strike role to varying extents. As such, they have the 'F/A' designation, like F/A-18 Hornet or they may have only the 'F' designation like the F-35 Lightning II in spite of the fact that it was designed to replace (among others) the A-10, an attack aircraft.

"CF-1 flight test" by Andy Wolfe - This Image was released by the United States Navy with the ID 110211-O-XX000-001 Licensed under Public Domain via Commons.

Another notable attack aircraft with the F designation is the F-105, although this was designated before the United States Tri-Service aircraft designation system was implemented.

The F-105 and F-111 were called fighter / bombers. During development the F-111was supposed to be a fighter for the navy, but that didn't work out, so the navy dropped out. According to Wikipedia ''A" was used in the air force designation system to denote amphibian.

The F-111 is a bad example, as it was originally designed to be an air defense fighter, a version that ended up not being built and the Navy buying the F-4 instead for that role (and eventually the ground attack role as well at times).

FWIW, the S in JSF was "Strike" which is another term for "Attack" in military aviation jargon.  F-35 is the JSF.

The F-117 name is unusual in two ways. Not only does it use the letter F, but it continues a numbering scheme which had been abandoned long before. The "century series" fighters were designed in the 1950s, and in 1962 the US introduced the Tri-Service aircraft designation system, which re-started the numbering. The first of those produced in sizable numbers was the McDonnell Douglas F-4 Phantom II, which was initially named F-110 by the US Air Force.
Both the fighter designation and the numbering were used as means of obfuscation. While the early work on Have Blue was made public, all work after 1976 was highly classified. Similar tricks were played before: The Me-163 was initially a competing design for a call by the RLM for a short-takeoff airplane which was won by the Fieseler 156. Only when design on a rocket-powered interceptor started was the number re-used to divert attention away from it.
The F-117 was intended as a strike aircraft from the outset. To be used as a fighter, any modern aircraft would need a powerful radar and external stations to carry air-to-air missiles. Both would had completely nullified the low signature of the F-117. Note that the F-22 carries a special version of the AIM-9M Sidewinder, called Boxoffice, with shorter span fins to enable them to be carried internally, thus preserving the low-observable characteristics until shortly before the missile is launched.

The F-22 uses the AIM-9X, which incorporates design features of the Boxoffice project. Boxoffice variants of the M never made it to full production before the Joint Chiefs directed the efforts toward AIM-9X.

@KeithS: Wrong, the AIM-9X is just being certified and will not go into service with the F-22 until 2017/18.

Whether the F-117 was "intended" as a strike aircraft or not, and despite some comments about it needing external stations, it did and could fire just about anything in the US arsenal including a 20mm cannon.
I was there, I worked on it as journeyman A,B & C shred avionics tech.  The bomb bay doors, where the normal GBU-27 were deployed from, had extendable/retractable stations.  It's actually how we loaded the planes on the ground. Prior to taxi, we the techs or chiefs would retract the stations, and close the doors. At one point in our history we pretty much disabled and removed most of those capabilities, but we did have it.

This was done to confuse the USSR, in case of an information leak while the F117 was still in development.
There were two parts to this simple deception:
The F designation, indicating fighter, would have the Soviets reading any leaked information as if it were intended for a state of the art fighter.
The century designation (117) was intended to get the Soviets to think that this was an older design that was abandoned. The USAF had rolled the numbers over into the F4/F5/F15/F16 over ten years before the F117 began development.
It helps that the USAF is normally very strict about aircraft designations. F means fighter... period. The only other case of a strike aircraft getting a F designation was the F111, and there were some thoughts about it taking on a fighter role when it was designed.
The F117 was always an attack/strike aircraft. Give the tech of that time, it didn't make sense to put that sort of design on a fighter that will be engaging other fighters. The odd shape meant that the F117 wasn't particularly maneuverable, and that it wouldn't be able to go supersonic.
Better to employ early stealth on strike aircraft, where maneuverability and high speed aren't a major concern, but not being detected by the radar that the USSR based its entire air defense network on, would be a huge boost.


text:
Why is the IFE only available after take-off and shut off before landing?
Every time I fly in an airplane that has an In-Flight Entertainment System (IFE) that consists of screens embedded in every passenger seat it is only turned on a few minutes after take-off and turned off a few minutes before landing? Why is this?
My limited knowledge of the system tells me that it should not interfere with any of the other on-board systems and thus it should not matter whether the system is running or not. The only reason I can think of is safety, similar to the question of why lights are dimmed during take-off and landing.

Not all IFE systems must be shut off, some of the newer ones can be used while on the ground.

@GdD Oh! I did not know this. All the airplanes that I've flown in before always turned it on only after take off.

They do this for the same reason they make you put away all electronic devices during that period of flight. The first minutes of a flight (take off and climb out) as well as the last few (approach and landing) are the most critical and statistically the time that if things are going wrong they will go wrong. As such airlines shut down the IFE systems and ask users to shut down personal devices so that they can hear, and more importantly respond to any emergency directions that may be given during this critical time.

2019 Update: Since this original answer was posted some airlines now allow the use of "small personal electronic devices" in all phases of flight.

Sounds reasonable. Though I wonder why they can't turn it on before the take off. Taxiing to the runway can sometimes take a full hour and it would be nice to have some entertainment for that time :-) Shutting it off just before take-off or simply pausing it would satisfy the safety measures.

Excellent point! I have added a link to at least one airline that considers taxi a critical time as well.

Very informative, thank you!

@Dave The FAA itself considers taxi to be a critical flight phase, at least for the purposes of their sterile cockpit rule. See 14 CFR 121.542.

Except you don't have to put away all electronic devices during take-off or landing. At least in the UK and the USA, they're happy for you to use things up to the size of a tablet during any phase of the flight, though they ask you to pay attention to the safety briefing and so on.

@DavidRicherby that is actually a fairly recent change in regulations (perhaps even since I wrote this original answer), ill edit to reflect.

@Dave Yes, I should have mentioned that it's recent. I'm not sure if it's more or less recent than your answer. My guess would be that it was about the same time.

For what it's worth, most of my experience over the last few years is that they don't do this, though exactly when they turn it off varies a bit from one flight to the next, even on the same carrier. I've watched movies all the way to the gate on Delta multiple times, though. Delta almost always seems to turn on the IFE prior to takeoff.
That said, after landing, often the flight attendants will start making announcements as soon as the aircraft has slowed to taxi speed. Most (all?) IFE systems I've encountered pause and lock out the IFE while announcements are in progress, so, in practice, it becomes difficult to catch much more of whatever you're watching after landing, due to the IFE system being locked out until they stop making announcements. Similarly, the IFE systems are generally locked out during pre-flight safety briefings (and often are used to deliver the briefings) during taxi-out.
One rather unfortunate consequence of the lock-out during announcements is that a lot of the flights I've encountered on multiple carriers play music or some such thing over the PA during boarding, which unnecessarily locks out the IFE systems. Similarly, international flights often make announcements in multiple languages and the IFE gets locked out for everyone for all of the repetitions.


text:
At what point are the pilots allowed to communicate with the ground crew whilst they are still in the cockpit?
The question comes from this comment by Federico

you cannot be directly connected with the ground crew while the aircraft is autonomously moving (and pushbacks are not used for gate alignment)

Is he saying pilots aren't allowed to talk to the ground crew until the plane is no longer under their control?

Allowed? They are always allowed! This question comes from a misunderstanding and should a be comment under the related post (asking for clarification).

That comments says “cannot be directly connected”, not “cannot communicate”. Pilots are allowed to communicate over radio with whomever they need to. It is just that the guy on the ground can't have their earphones plugged directly into the external connector on the aircraft as they do during push-back.

Here is a great YouTube video of a pushback. The engines are started while still attached to the tug. But note that the tug and all personnel are clear before they power up and start taxiing. That would run the risk of someone being sucked into an engine.

This is what could happen if personnel is not clear.

@mins Unfortunately without enough reputation you are unable to make comments on posts

@JanHudec I was unaware that the ground crew literally plugged a cable into the plane to communicate, I would've assumed it was over some wireless radio frequency being in 2017 and all.

@Ksery, yes, they do. This has the advantage that the communication is properly duplex and can't be interrupted by unrelated calls like on the radio, which uses shared frequency. This is big advantage especially during push-back where if something goes wrong, the walker needs to tell the pilots to slam on the brakes quickly. But it may only be used when the plane is stopped (loading, refueling etc.) and during push-back, but not while the plane is moving under its own power. That's all the comment says.

no longer under their control

is a strange phrasing, the aircraft has to stop sooner or later, do you define a parked aircraft "not under the pilot's control"?
I'm saying that the ground crew does not generally connect the external "headphones" while the engine is used to power the aircraft, the connector is near the nose gear well and the cable is generally not overly long, to avoid the grond crew from being ingested by the engine, they generally wait until it is safe.

Honestly, I'm not 100% aware of how the procedure works once a plane has landed and taxi'd to a gate, I wasn't aware ground crew literally connected their headphones with a wire, I assumed it was based on some radio frequency that both parties (cockpit and ground crew) connected to (wireless). I also would define 'no longer under their control' as at any point when the ground crew would be able to control the movement of the aircraft, i.e pushing or towing it out of the gate or connecting the boarding bridge.


text:
When do I need to worry about a course deviation indicator reverse sensing?
I understand that if I am tracking a radial inbound I will set that radial as the reciprocal so I don't have to worry about reverse sensing. On an approach however, do I need to set the CDI to the reciprocal? If I go missed and track the same course outbound do I then leave the CDI on the same heading?
On this approach plate, which was talked about here, why is the back course on two different sides (one is shaded on the left, one is shaded on the right)?

How does a front course and a back course affect your CDI settings?

Are there any difference between these two CDIs?

(And also, why is it labeled a VOR receiver? I thought both of these instruments were called Course Deviation Indicators.

On this approach plate, which was talked about here, why is the back course on two different sides (one is shaded on the left, one is shaded on the right)?

The 108.5 IPKN localizer is there only to mark safe path out for missed approach procedure and is intended to be flown outbound, so it has the signal swapped so it indicates normally when flown outbound.

How does a front course and a back course affect your CDI settings?

It does not affect settings of CDI at all, because ILS does not take the OBS course into account. It indicates left when it senses more of the left signal and right if it senses more of the right signal, independent of anything else.
What it affects is:

The left signal is on the left when flying the normal direction and this applies both before and after the transmitter. So when you fly in opposite direction, the left signal is on the right from your point of view, causing the reverse sensing.
With HSI you can compensate for it by selecting the normal course in the OBS. The instrument then still senses the same, but the instrument is now upside down, negating the reverse sensing.

Are there any difference between these two CDIs?

No, since these are both CDIs.
There is a difference if you have HSI:

The difference between CDI and HSI is that on CDI the needle always moves left-right, indicating whether your position is left or right of the selected radial (via OBS for VOR, but fixed for ILS) independent on direction you are flying. So if you are actually flying the opposite direction, you must steer the other way.
In contrast the inner part of HSI rotates with compass, always pointing the arrow in the selected direction. This has two advantages:

you see how your direction of flight relates to the selected heading and
when you are flying in the opposite direction (backcourse), the needle is upside-down, negating the reverse sensing.

And also, why is it labeled a VOR receiver?

Because it is (also) used to display VOR signal. Both CDI (where the needle always moves left-right) and HSI (where the inner part with the needle rotates with compass) are VOR indicators.
Note that almost always the same tuner is used to receive VOR and ILS signals and the same instrument is used to display them though the processing logic is different (VOR detects phase while ILS detects differences in the depth of modulation from the two signals).

And also, why is it labeled a VOR receiver? A VOR receiver can receive both VOR and LOC.


text:
How does flying below the freezing level in a thunderstorm help avoid the most critical icing?
AC 00-6A (Aviation Weather) Chapter 11 states:

Do's and Don'ts of Thunderstorm Flying

To avoid the most critical icing, establish
a penetration altitude below the freezing
level or above the level of -15 C.

I'm trying to understand how flying above an altitude where -15 degrees Celsius occurs would help me avoid icing. I'm also trying to understand how flying below the freezing level would help me avoid icing. What is the explanation?

Isn't it crazy that with all the technology we have and advancements we've made, we still have to worry about a little water getting in the wrong place at the wrong time?

@corsiKlauseHoHoHo No.

The icing occurs due to the presence of supercooled liquid droplets. From FAA AC on Icing:

The condition most favorable for very hazardous icing is the presence of many large, supercooled water drops.... Thus, heaviest icing usually will be found at or slightly above the freezing level where temperature is never more than a few degrees below freezing.

For ice to form on the aircraft, the following conditions are required:

The presence of liquid water i.e. rain or cloud droplets.
The temperature at aircraft structure should be at or below freezing point.

Now, as the altitude increases, temperature decreases. Using this, the best way to avoid these conditions would be to either:

Fly above the altitude where the temperature is below -15 degrees, where supercooled water drops are not present (they would've frozen already), or
Fly below the altitude where the temperature is above freezing point, where the water droplets, if present are not supercooled.

Temperature decreases with increasing altitude.
The “freezing level” apparently means level, or altitude, where the temperature equals freezing point. Therefore below it, temperature is higher than freezing point and so no freezing occurs (though keep in mind that temperature above the wing is slightly lower than ambient due to adiabatic cooling, so icing actually starts at temperature slightly above 0°C).
Now to explain the altitude above -15°C we need to mention that icing occurs when the atmosphere contains “supercooled” water droplets that are still liquid at temperatures below freezing that can freeze when they come in contact with the aircraft. But at temperatures below -15°C most droplets have already frozen into tiny ice pellets. Since those won't stick to the aircraft, icing decreases again when it's too cold.

Temperature usually decreases with increasing altitude.  Occasionally, there's something called a temperature inversion, where temperature increases with altitude.

Icing on aircraft relies on liquid water hitting the airplane and freezing onto its surfaces.
The typical outside air temperature range for ice formation is from -20 °C to 2 °C. Water in the atmosphere can become supercooled to below 0 °C, and then freeze into ice upon contact with the aircraft. Below -15 or -20 °C, the water is much more likely to have frozen into ice particles, which do not accumulate as ice would. This is why the colder temperatures will prevent further ice formation.
If the temperature is above 2 °C, it will not be supercooled. While it's possible that the water could freeze to a cold surface, the wing will typically not remain significantly colder than the surrounding air.

I'm also trying to understand how flying below the freezing level
would help me avoid icing. What is the explanation?

Even on a warm summer day, temps can be below freezing high up in a towering cumulus cloud. Playing around the edges of a towering cloud, I've had the yaw string of a sailplane get frozen to the windscreen, when it was 85 to 90 degrees or more Fahrenheit on the ground below. Air temps at that level were undoubtedly below freezing. The people on the ground were not experiencing icing, because they were well below the freezing level. They may, however, have been below the sweating level as well.
Perhaps what you are not understanding is that "below" is a reference to altitude, not temperature.


text:
What are the most common cockpit system and instrument failures in small airplanes?
What could one expect to be the most common cockpit system and reliant indicators to fail in a small airplane like a Cessna 172 or Piper Archer? Would gyroscope failures or vacuum pump failures be one?
Edit: This question is different than this one: What component of an aircraft is most likely to fail? because I am not asking about components, I am asking about systems and instruments.

Re. your edit, what is the difference you see between a "component" and a "system" or "instrument"?

I have an FAA magazine that I could answer this question with.

From the duplicate, according to the selected answer it's clear that the gyros and vacuum pump are not particularly failing components. Do you want to know something else that is not in the duplicate? Please clarify your question.


text:
How do redundancies work in aircraft systems?
I've read about many aircraft systems having redundancies. For example, both Boeing's 787, and Airbus's A380 have a triple-redundant FCS. In this case, it makes sense to act upon the majority's decision. If all three components disagree, how is it decided which one wins? Is there any record of disagreement amongst redundant systems in airplanes?
The primary differences I have compared to the question about Airbus's control systems (How does the Airbus flight computer's voting system work?), is verification for non-deterministic systems, as well as optics for visibility. Given that the entire systems run in parallel up until the verification component (from sensor input, onwards), there will be some, obvious delta in their "decisions." -- Although, most of the time, this isn't perceivable, is there any visibility into what's going on with the FCS?

Relevant question on the difference between fault tolerance and robustness on engineering.se.

It helps to realize that components have fairly specific failure modes. Probably the two most common are "no output" and "random output".

Related: Why are critical flight computers redundant?

The question "How do redundancies work in aircraft systems?" is a good one, but several questions adress the software aspect of this question on this website. You should have focus your question on other redundant systems (hydraulic, electric,...).

Triple-redundant systems have four independent members, so if one fails, a two-to-one vote of the remaining three is still possible.
One system    = No redundancy.
Two systems   = Simple redundancy.
Three systems = Double redundancy.
Four systems  = Triple redundancy.

However, often the term "triple redundant" is used when the system has only three members. And if all three disagree, two of them must be faulty already, so this is a case of multiple failures.
In case of Airbus, besides the three main FCS computers two backup systems are available to help isolate the faulty one. Inside, each has two channels which run software written by a different team, run on a processor different to the one in the other channel and which are supplied by two different suppliers.  This helps to reduce systematic errors, when all together would fail because of a common malfunction.

Airbus flight control system diagram (picture source). Only the first system of each is expanded; the boxes for system 2 and 3 look the same on the inside. Input is from the side sticks, and output goes to the hydraulic valves at the control surfaces. The voting happens first in the comparator step (between the two channels) and again in the filters (between the single systems).

Yeah, but if there are two, or four systems, how do you determine which system to follow?
In addition, is there a mechanism to signal whether the systems are "disagreeing"

@SargunDhillon: In four systems two must have failed in the same way to make a vote impossible. This is unlikely enough to be deemed acceptable. The responses from the FCS computers are compared, and only if they are identical and take the same time to be reached they are accepted.

Is the comparator running on a third separate microprocessor? I feel like having redundancies is actually somehow just moving the problem further away. What if the very last filter actually fails and eg outputs a logical 1 although  it was supposed to output a logical 0? Yes you could have a redundant filter. But then again, you are just moving the problem further away. At the end of the day there will only be 1 device which eg will compute the final majority vote to say "OK, move the aileron". What if that one fails?

@traducerad: All channels work in parallel, so if one gives a faulty signal, it is either switched off (comparator stage) or its input is diluted by the other, properly working channels. A single failure will not affect safety in a "digital" way (0 instead of 1).

Your question also makes the implicit assumption that redundancy involves multiple systems trying to come up with the same answer.  This isn't necessarily the case.
I've worked on a multiply-redundant hybrid electric vehicle controller.  These things are drive-by-wire as far as go/stop are concerned, so a lot of the same safety principles apply.  A primary control system ran the whole thing, managed emissions, best driver response, and all that good stuff.  A secondary system had a simpler model of how things should behave, checking for when the primary system was outside some tolerance of its simpler model for some time.  Because it was a simpler model, testing could be more complete.  And a tertiary system checked that the primary and secondary systems were coming up with valid behaviour, using an even simpler (and hence even-easier-to-validate) model.  For added fun, the secondary and tertiary systems periodically injected spurious faults and checked their fault-reporting mechanisms were detecting them correctly, so they knew a real fault would trigger correctly.
In this case the fallback behaviour was simply to pull the plug on the car if the primary system went dodgy, because for a car that's reasonable.  In principle though the secondary system could have been the basis of a perfectly competent fallback strategy if we'd thought it was necessary.

In general, the number of redundancy indicates the number of independent members i.e. triple redundancy indicates three independent members, while a quadraplux redundancy indicates four independent members. A triple redundant system used in Flight Control Systems would look something like this:

Image from A Novel Family of Weighed Average Voters for Fault-tolerant Computer Control Systems by Latif-Shabgahi et.al.
There are two ways the 'Voter' can determine the output from redundant systems:

Agreement based voting system produces an output only if a certain number (e.g. majority) of the voter inputs agree. This is usually the method used in Flight Control Systems (like Airbus).
The other system produces an output even if there is no agreement between the redundant systems, usually by 'combining' (for e.g. averaging) selecting one of them based on some metric (for e.g. median).

Consider the air data input in case of Airbus: there are three Air Data and Inertial Reference Units. The input of the 'voter' (each flight computer) comes from all three ADIRUs. The FCS needs two matching (with some allowed variation) values to generate output. The Flight computers degrade to alternate law or direct law under two circumstances:

More than one unit fails.
No two units agree.

In case of Air France flight 447, the FCS reverted to alternate law due to temporary inconsistency between the measured speeds (likely due to obstruction of the pitot tubes by ice crystals) of two ADRIUs. The another one had already been diagnosed as faulty and excluded from consideration. This and the consequent pilot actions resulted in the crash.


text:
Can planes take off from a treadmill?

Note that this is more of a Physics question than an Aviation one.
This topic has been extensively covered on the Physics Stack Exchange site, and the answers there are recommended reading for anyone curious about the forces and physics involved.

If you had a giant treadmill that was an infinite plane, had an infinitely adjustable speed, and could tread along any of its dimensions, could an airplane take off from it?

http://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/

Oh no, not the treadmill!

Ahhhhh! Well we had to have it on here. ;)

Someone did build a giant treadmill to test this. See the Mythbusters video below

It should be noted, that the xkcd version makes the threadmill actively match the wheel speed, leading to conflict, while this question just assumes threadmill that is free to move, which has clear answer.

The treadmill is irrelevant.  All it can do is spin the wheels.  It cannot move the aircraft.  Myth Busters should be considered entertainment and not serious scientific enquiry.  They did a lousy job with this one and demonstrated, well, that an aircraft can fly.

@Simon - I don't think I understand your point that all the Mythbusters proved was that the aircraft could fly. They put an aircraft on a big sheet of canvas, had Jamie pull the canvas in one direction while an ultralight accelerated in the other. Unless you wanted them to build a motorized conveyor-belt runway on a TV show budget I don't think they could have illustrated the concept any better. You are either discounting the possibility that it wouldn't have taken off out of hand (when a lot of people were arguing that point of view) or you don't understand what the experiment showed.

Everyone collectively went "Oh god, not this one" because this same question has sparked some intense debates in the past. Aircraft rely on airflow over the airfoil (wings/tail etc) to produce lift - which is independent of the movement of the tires. This means that with enough air going over the wing, the aircraft will fly even if it isn't moving forward at all relative to the ground.
This is why aircraft on ramps at airports need to be tethered to the ground. This isn't only to prevent them from rolling around, but from taking off should the air flow get fast enough over the wing.
If you are interested in an entertaining segment, the MythBusters did a fairly scientific experiment of this.

 Although it isn't perfect, I think it does a good job of explaining the concepts.

It isn't that it won't be moving in reference to the ground, it's that the treadmill can't stop it from moving in reference to the ground.

All that Mythbusters video did was prove that the wheels on the plane could handle rotating at twice the RPM necessary for the plane to take off.

@FreeMan: The wheels are free-wheeling and are essentially not connected to the plane. Think of it like this: if you lie on your back wearing rollerblades, can I lift your leg by spinning the wheels? Your're saying I can.

@slebetman not sure I follow you. The plane was moving forward at X knots, while Jamie pulled the "conveyor belt" backwards at X knots. The wings generated enough lift for take off, but the wheels spun at 2X. Therefore, all the video proved was that the wheels could handle spinning at 2X, where they would normally rotate at X. I fully understand that take-off has nothing to do with the rotational speed of the wheels and that's why I indicated the experiment didn't prove anything else.

@FreeMan: The wheels being able to handle spinning at X speed has nothing to do with keeping the airplane from moving forward. The wheels are basically acting as roller bearings. The only way to stop the plane is to make the wheels explode (which may be possible to do with a threadmill in theory). There was a also an answer in the physics SE that pointed out that the rotational inertia of the wheels can impart a tiny force on the plane. But it would take the wheels moving at or beyond the speed of light to have any noticeable effect.

@FreeMan - except that you just described the requirements for the plane to take off from a treadmill. The wheels simply have to be able to spin freely at double the takeoff speed without the axle's friction at the higher speed slowing the plane below the forward airspeed needed to take off. Thus, they proved that an airplane can take off from a conveyor belt.

@slebetman I think we're saying the exact same thing, just from opposite directions, so I'll agree to disagree on the wording of our statements while agreeing to the principle that the conveyor belt under the wheels has negligible impact (at most) on the plane's ability to take off.

The "experiment" that would prove the conveyor belt has nothing to do with take off would be to have the pilot advance the engine to cruise idle while Jamie pulls the belt in the direction of take off. The wheels wouldn't roll at all, but eventually enough wind speed would be generated, the plane would take off, and, with some skill, the pilot would climb out, albeit slowly.

Yes.
Airplanes get their thrust by using the air. The wheels are not powered. The drag from the wheels will limit how fast the treadmill can go before the plane won't be able to take off anymore.
It's simpler to understand if you pick a different frame of reference. Assume the treadmill is standing still but the air is moving around it in any direction with any speed.
Notice I just described a windy day.

Has there ever been an airplane that has flown away, or been lifted away on a windy day?

@SargunDhillon Yes. –slebetman's YouTube link.

The windy day argument is somewhat flawed.  If the wind is blowing along the airfield the planes will take of into the wind.

@Taemyr there is nothing stopping the plane from turning on the infinite length conveyor and turning into the wind.

@ratchetfreak I would say that the question implies that in that case the threadmill would run the other way.  Essentially turning the wind.

@Taemyr it's not impossible to take off with a tail and/or crosswind just more bothersome. (remember the treadmill in the question is infinite in all sides)

@ratchetfreak Agreed, the question is equivalent of taking of from an infinitly long airfield with an arbitrary strength laminar tail wind.  I also agree that this would be possible.  -  However I think those that claim the aircraft could not take off from the threadmil would also claim that the aircraft could not take off from this infinite airfield.  In the actual world two things would prevent take-off, runway length and turbulence.

@Taemyr The treadmill has nothing to do with where the wind is coming from.  This is dependent only on "the wind" (which the OP does not mention) and the aircrafts thrust/heading.

This question is at the best, ambiguous. There can be both yes and no answers based on what is being done with the aircraft and treadmill. The point is that for an airplane to lift off, there should be sufficient airspeed. If there is no wind, there airspeed is equal to the ground speed
Assuming that there is no wind (into or against the aircraft), there are two possible solutions.

If the airplane is stationary relative to the ground, it won't take off (as wind speed is zero).
If the airplane moves relative to the ground (with sufficient speed), it will take off.

Assume that we have a jet airplane (just for sake of argument) and some one pushes the throttle and it begins to move forward. Now, as the treadmill has an infinitely adjustable speed, we can have three conditions:

If the treadmill speed is zero, the airplane will eventually generate sufficient lift and take off.
If the treadmill speed is adjusted such that the airplane is kept stationary relative to the treadmill, the airplane will take off (as it is moving with respect to ground, and so has some airspeed).
If the treadmill speed is adjusted so that the airplane is kept stationary relative to the ground, the airplane cannot takeoff, as the ground and air speeds are both zero. Note that in this case, the aircraft speed relative to the treadmill is twice that of the speed at which the treadmill is being operated.

If there is a wind, the required ground speed can be adjusted accordingly, but the principle remains the same. For example, if the wind speed is equal to the airspeed required for take off, the airplane will lift off even though it is stationary with respect to the ground.
Again, the important concept here is airspeed. It does not matter if the aircraft is on a treadmill, train track or runway.

Your last bullet point is flawed, and is the whole problem with this stupid "riddle". The treadmill has absolutely no way to overcome the independent thrust generated by the engines - it can not prevent the aircraft from moving relative to the ground, and therefore relative to the wind.

@Dan Till the airplane takes off, the wheels are on the ground. If the treadmill speed matches the rotational speed of the wheels (note the treadmill has infinite speed and nevermind the rolling friction), the plane could theoretically be stationary relative to ground. This problem is because the question is quite open ended without any bounds.

@aeroalias How? The physics don't match - I guess at a certain speed you might have a wheel bearing failure, but while those bearings are functional there's almost no relationship between forward thrust and the rotation of the wheels.

@Dan Please do see the same question in Physics Stackexchange for a much clearer explanation of what I'm trying to say.

This issue is that #3 isn't possible. No matter how fast the treadmill is running, the tires on the airplane are free-wheeling so simply spin faster while the aircraft continues doing what it was doing before (accelerating if it is under power). Put another way, if the engines were off and you start the treadmill, (assuming that the brakes are not set), the tires would spin but the aircraft would remain stationary. Note that for clarity this ignores the friction between the axle and the wheel hub and that eventually it would slowly start to move, but this is overcome by thrust in the question.

What everyone misses is that the treadmill, and what it is doing, is totally irrelevant.  The only thing it can do is spin the wheels (ignoring friction, which is a rounding error in the forces involved).  The only thing that matters is air over the wings and the only way that can be generated (assuming zero wind) is for the aircraft to produce thrust in the normal manner.  The first half of your answer is correct.  The second half is wrong.  The treadmill cannot move the aircraft.

This is the only answer that goes into details about the different legitimate interpretations of the question, therefore this is the only complete answer. That said, there is a mistake in the third point. To keep the plane stationary relative to the ground, the relative speed between the plane and treadmill is much higher than double. The treadmill can only slow down the plane through parasitic rolling resistance in the wheels. IE friction.

Because of parasitic loss, if the plane is putting out enough thrust to travel at 100kph, it will only travel at 99kph if on the ground. If the treadmill is going in the other direction at 100kph, it would only slow the plane down by 1 extra kph. It would be the same as the plane putting out enough thrust to go 200kph: it would only go at 198 kph on the ground. You need the force the treadmill is applying to the plane to equal the force the plane is applying to the air. And your treadmill is only about 1-5% efficient at slowing down the plane.

@Shane  As it is, the question can only be answered by applying assumptions and constraints.  If you introduce friction, momentum and rolling resistance into this hypothetical nonsense, then it becomes totally unanswerable unless emperical bounds and constraints are added to the question.

Yes. It does not actually matter much which direction and how fast the treadmill would rotate; the aircraft will take off.
The only requirement for generating lift is to move through the air sufficiently fast. The speed is created by thrust. And the thrust of the aircraft engine does not depend on the ground speed ("ground" would be the surface of the treadmill for this case).
The treadmill can only affect the ground speed so would have no effect on the engine thrust. Hence it would not have significant effect on the air speed either, unless through the friction forces in the bearings of the wheels. I assume that these forces are small in comparison the power of the engine.
The only chance, as the aircraft chassis is the designed for the limited ground speed only, the treadmill may prevent take off by rotating in the opposite direction fast enough to cause the chassis to collapse.

If we ignore friction, the treadmill would affect neither ground speed nor airspeed, unless you're defining 'ground speed' to be speed relative to the moving treadmill surface. If you're defining 'ground speed' as being relative to the Earth that the treadmill is sitting on (i.e. normal definition of ground speed,) the ground speed would differ from the airspeed only by whatever the wind happened to be at the time.

What forces would cause the chassis to collapse?

@Octopus Friction in the wheel bearings leading to a torque on the landing gear struts.

"Ground speed" I mean relative to the surface of the treadmill (the "ground" the aircraft is taking off from). The treadmill may force the wheels to spin faster than they bearings could hold by moving fast enough in the opposite direction.

@DavidRicherby Not to mention friction in the wheel bearings and friction on the tire surface (both with air and with the treadmill surface) likely setting the gear on fire.

Theoretically yes. In reality it depends.
Theoretically
We don't account for friction in the landing gear wheel bearings or between the treadmill and the wheels. This would mean that with the plane just sitting idle, if the treadmill moves the plane will remain still. You can try this by putting a toy car on a piece of paper. If you jerk the paper back and forth the car doesn't really move. The only reason the car moves is because of friction. If you eliminated friction in the wheels the car wouldn't move at all. We've now established that the moving runway has no influence over the plane. The pilot is free to start the engine and take off.
In reality
The real answer depends on the design and limits of the plane/treadmill:

In real life there is friction in the landing gear. There are limits to how fast the wheels can spin before they fail. But, there would also be a limit to how fast the treadmill could go.
There are limits to how fast the treadmill and plane can accelerate and change directions. A pilot may be able to get the treadmill going in one direction then turn around and take off in the other.
A very large treadmill moving at high speed would create wind. A strong enough wind can allow a plane to take off even though it's standing still.

 The amount of wind generated would depend on exactly how smooth the surface is and how large it is.

"or between the treadmill and the wheels."  If you don't account for friction between the tires and the treadmill, the wheels won't even turn. The aircraft would just skid across the surface of the treadmill.

I had a thought here:  If we are considering a perfect treadmill and perfect wheels/bearings on the plane it doesn't take off.
The plane starts to roll.  The treadmill matches the wheel's speed, but this simply spins the wheels faster--so long as the plane is rolling the treadmill is in a never-ending race against the wheel.
Since we are looking at a perfect system this proceeds without limit and infinitely fast--the treadmill (and wheel's outer edge) is going to be approaching lightspeed.  The mass grows without limit, the plane is too heavy to take off.
In the real world with imperfect systems something has to give.
1)  The wheels have a maximum speed.  Exceed that by too much and your landing gear explodes.  The plane pancakes onto the treadmill, the friction is too great for it to overcome, it's tossed backwards and then stops.
2)  The treadmill has a maximum speed.  If the wheels can survive takeoff velocity plus this speed the plane takes off, else #1.
3)  The treadmill has a finite acceleration rate.  The plane very well might take off before the treadmill has built up serious velocity.

authors: مروان بن حسين


text:
Is the aft position the ideal place to fly a large aircraft from?
Cockpits are at the front of the airplane, and they have pretty limited visibility. Pilot windows can actually open to help with some of this. We know that there are a ton of tricks that are done to assist pilots in navigating their aircraft with the limited visibility that the cockpit provides. The A380 has multiple cameras to assist in visibility. On the ground, there are people to assist in taxing the aircraft.
Has there ever been any research into a flight engineer, or pilot flying an airplane with a VR headset?

. . .What would you do when the VR headset fails?

@voretaq7 - Ideally, switch back to looking out of the glass cockpit? I'm not proposing killing cockpits as they are today, but it'd be interesting to have a VR view in addition.

Yes, it's where all the controls are.

As always with these kind of question, you should state what problem it would solve.  I am not aware of any problem with commercial traffic and visibility.

Possibly relevant - Windowless cockpit patent article

The only direction a (fixed wing) aircraft can move using the flight controls  is forward. So, it is the forward visibility that is important. The front of the aircraft is the best place for this purpose. The limited visibility in other cases are reasonably over come by using extra cameras or marshalers and markings in the ground.
The Airbus A80, for example, has three cameras, one each in the nose, belly and the tail,which can be viewed in the screen by the pilot.

Source: onemileatatime.boardingarea.com
The closest any aircraft has come to be flown using a VR headset is the  AN/AAQ-37 electro-optical Distributed Aperture System (DAS) which is being used in the Lockheed Martin F-35 Lightning II.
The EO/DAS consists of six high resolution Infrared sensors  flush-mounted around the aircraft to afford 360-degree, spherical (4$\pi$ steradian) coverage, basically allowing the pilot to see through the airframe.

Source: alternathistory.org.ua
Each of these sensors collect data in different directions and the on-board computer 'stitches together' these images (and data from EOTS and Radar) and projects them onto the pilot's Helmet Mounted Display System (HMDS), making it the only modern combat aircraft without a HUD.

Source: brucesterling.tumblr.com
However, there are issues with using these in a commercial airliner:

They are costly- around $400,000 (some reports say ~700,000) apiece and non-interchangeable; while this (situation awareness) could be priceless in combat scenarios, I don't see commercial airliners shelling out this amount of money to replace a system that works pretty well.
The amount of computing power required is enormous. Reportedly, the DAS system collects about a terabyte of data per hour. For aircraft which fly most of their time in a predetermined altitude and heading, this amount of data flow and computing power is not required.
Note that the system is not yet fully operational. There are issues with the helmet like latency, 'green' glow, jittery image, resolution issues etc.- reportedly solved, though some pilots say they don't use this system very often.

Ships mostly go forward and yet are often piloted/steered from the back or the middle. Not saying this would be a good idea for airplanes but explaining the difference could be interesting.

@Relaxed Cars, trucks and trains are also driven from the front. Ships are the exception, because it usually makes most sense to have the bridge where the engine is, so your engineers aren't a hundred metres away from the people who are controlling the vehicle.

@Relaxed Just adding to David's point- Cruise ships do have their bridge in front, as do the RN's new Queen Elizabeth Class Carriers. In cargo ships, it makes sense to have bridge close to engines as the crew complement is pretty small.

@aeroalias Yes indeed but my point was that it's worth adding that to the answer…

Interesting to claim that the HMDS is not a HUD (not sure if that's your claim, or one made by others you're just repeating). I'd call it a HUD that just happens to always be in front of your face. Just a matter of semantics, though.

@FreeMan Agree its just a matter of semantics; I'm not claiming anything. Even the manufacturers use these terms interchangeably. Just that it helps me separate what I think are different in the way they interact (with pilot). Of course I can be wrong.

1) The helmets include some comm gear, integrated NVG's, and must function at 9+ g's. Presumably a commercial model would cost less. 2) 1 TB/hr isn't that much. An iPhone collects up to 7 TB/hour (8 MP x 240 fps x 1hour). DAS is relatively "low def," being primarily the F-35's missile approach and warning system (MAWS). X-ray vision is secondary. 3) The new Gen III helmet solves nearly all the issues from the old ones; it looks like this. AFAIK, pilots prefer the Gen III over all previous helmet systems.

More info on helmet-mounted displays for the curious: http://www.f-16.net/forum/viewtopic.php?f=62&t=16223&start=585


text:
How does a (non-pilot) arrange a tour of an ATC facility?
Although the question has been asked for pilots, how does a civilian get access to a ATC facility? I regularly listen to Live ATC, and I'm incredibly curious as to what the inner-workings of a tower look like.
Even at that, is there a live video feed of ATCs?

The question you linked to isn't limited to pilots and it even mentions non-pilots. Is there something specific about access/visits that isn't already answered there? Your question about a live video feed is different, perhaps it should be the main question here.

For tours, call the facility and say you're interested in aviation, and would like a tour, same as pilots. Depending on the facility, business hour tours are preferred, though others will let you come any time they have available staffing to escort you around.
As far as I know, there are no video feed of any ATC facilities.

How do I get the phone number of Norcal TRACON? And SFO ATC?

@SargunDhillon: I typed "phone number norcal tracon" into google and found relevant information that may be helpful for you.


text:
In EASA, what is Panel 12? Is this just the same as Panel 10?
IN EASA safety approval guidance, particularly CM – SWCEH – 001
Development Assurance of Airborne Electronic Hardware, the entity charged with making sure that a system meets applicable requirements and guidelines (especially concerning safety) is called "Panel 10."
However, in section  5.2 "Background" there is a list of panels, and there, Safety is listed as Panel 12.  (Immediately after this list comes most of the assurance-related material, which extensively speaks of Panel 10.)  Yet no other mention of Panel 12 is made in this document.
Is this just a typo, where they meant Panel 10, not Panel 12?

Reading through CM-SWCEH-001, the Background section is identifying the EASA system panels, one of which is identified as the primary panel for certification requirements for the system being developed. My understanding of Panel 10 is that it serves as a secondary panel for some of those requirements.
The document defines Panel 10 as:

The EASA panel in charge of software and AEH aspects of certification.
  This panel includes at least one software and AEH expert (the
  coordinator) and, depending on the size of the project, may include
  additional software and AEH experts.

I would say that it is not a typo, with Panel 12 being the System Saftey panel.


text:
What is this red canister on the tail of the YF-22?
On the tail section of the YF-22 further back in the image below, there is a red canister. What is that canister for? I suppose it is for some flight testing purposes from what I can see.

I can't pinpoint the image source.

Similar system on a X-29: What is the 'arm' at the rear of the X29?

This is a good question, but a duplicate of the X29 question.

Same answer, but entirely different QUESTION. Not a duplicate at all. Different airplane, and the add-on equipment doesn't look all that similar, even if the purpose is the same.

IF the same answer can be used to answer this question, it is by definition a duplicate ... Two completely different questions, if both served by the same answer, are duplicates.

@CGCampbell I think the answers are similar, but not the same because the piece of equipment itself in question is different.

@CGCampbell The answers to two different questions being the same is not the same situation as an answer to one question including the answer to another. Both questions have the same answer, but those answers don't inherently answer the other question. You wouldn't (or, at least, shouldn't) close "What is the integral of x^2 from 0 to 3?" as a duplicate of "What is 5 + 4?" for example.

@reirab but here we are comparing "What is the integral of x^2 from 0 to 3?" to "What is the integral of y^2 from 0 to 3?". It is the same object, simply with a different colour and mounted on a different aircraft. Or we will have to accept all future questions asking about this same object when mounted on all aircraft that will undergo (or have undergone) this kind of tests.

It is a spin recovery/stabilization chute, installed during testing for high AoA test flights. The following image shows it being deployed on the ground.

Image from fas.org

Should it/ Is it usually deployed on ground or inflight?

@AlexS its an emergency system designed to deploy on command if the aircraft gets into a maneuver that the pilot cannot recover from during testing of the aircrafts envelope.

The canister contains a small parachute and either a spring or an explosive charge to kick the chute out when it is needed. It is a precaution for spin flight tests, that's why it is called a spin chute. See here for a video of the spin chute of the F-35.
I guess now I should explain how a spin chute works.
When an aircraft spins, it will rotate around a vertical axis which is somewhere between slightly ahead of the aircraft (regular spin) or near the leading edge (flat spin). The pitch angle $\Theta$ is nose down and the angle of attack $\alpha$ is high (around 45° in a regular spin, up to 90° in a flat spin), so the airflow over most of the wing and horizontal tail is separated. But the rotation, which is a mix of rolling and yawing, will induce an angle of attack variation over span, so a part of one wing will operate in the normal angle of attack range, where lift is high and drag is low.
Now we have a receding wing with separated flow, at high angle of attack, with low lift and high drag so the resulting air force vector R (green, below) points mostly into the direction of local flow (which is up). The other wing has partially attached flow, a lower angle of attack, high lift and moderate drag, so the resulting air force vector is almost perpendicular to the local flow, pointing up and forward. This difference in lift and drag propels the rotation.

Speeds and resulting force on spinning wing sections. $\omega_z$ is the rotation speed around the vertical axis, y is the local wing station and the green v shows the local flow direction.
Without the rotation the aircraft would immediately pitch nose down, pick up speed and can be pulled out of the resulting dive. With the rotation, however, we get an inertial pitch moment due to the fuselage masses. All parts of the aircraft rotate with the same yaw rate, and the centrifugal force from this yawing motion grows linearly with distance from the spin axis. This difference in centrifugal force along the lengthwise coordinate of the aircraft adds a powerful nose-up moment which in some cases cannot be overcome by the tail surfaces - remember, they have less effectivity in separated flow. Flat spins are almost impossible to escape from.
This can be tested in spin tunnels, but Murphy's Law is very relevant to spin tests. So it is a good precaution to add a spin chute for the first tests: If the aircraft cannot end the spin with control surface deflections, it is in a trap from which it can only escape with something that will add a strong nose-down moment. In the chaotic airflow behind a rapidly spinning airplane. That is why the spin chute is not bolted to the skin of the airplane, but sits in an elevated position hanging out at the back. From there it will not be caught by the tail surfaces but will work as intended in all conceivable situations (save an inverted spin ...)

1+ for actually explaining what a spin chute is for

In the last paragraph, didn't you mean a nose-up moment? The chute would have to be below rather than above the plane's COG to pull the nose down, right?

@Mels: No, airflow is coming predominantly from below, so the chute will fly above the aircraft and pull the tail up.

Ah, I get it. Thanks for the clarification :)

I'm very confused by the spin mechanics you've described. The plane starts with a pitch down (para 3), which is good (para 5), except for the rotation which causes pitch up (later in p5), which needs to be overcome by the tail (end of p5). If the tail cannot overcome the pitch up, you pop the 'chute which adds nose down (p6). Are you saying that the spin initially starts nose down, but then converts to nose up (because physics), and if that nose up is not counteracted by the 'chute, the ground will handle that task for you?

@FreeMan: I tried to describe a spin as briefly as possible and left out many details. The pitch attitude is always nose down (regular spin) to horizontal (flat spin). High pitch is better because it is easier to end. This is all true for a fully developed spin, so there is no sequence. A description of the development of the spin would include the incipient spin, which is characterized by pitch oscillations and rapidly changing flow conditions. I felt it would be too much to include here. Then we should also talk about oscillating spins ... why don't you ask specifically?

@PeterKämpf because I don't want to give you an excuse to write us a text book.  :)  I just wanted to be sure I understood the basics of what you were explaining - I think a full description would go beyond my ability to comprehend. (There's more than one of your answers that I've read through, nodded my head, and said, "Yeah, I get that (no, I really don't)")

@FreeMan: You know, once at University I remarked to a friend "I am too stupid to understand this", whereupon he replied: "No, the professor is not able to explain it well enough. It's his job, after all". I had to agree. Now I want to explain things well enough so anyone interested can follow, and I need your help to improve my answers. Please point out where you got lost and give me a chance to rewrite stuff where I expected too much and need to slow dow.

Thanks, Peter, I'm sure I could understand it if I wanted to, I'm just not interested enough to invest the time & mental energy to make heads or tails out of all the math you tend to post. Getting the general understanding is more than sufficient for me (as a non-pilot).

For what it's worth, as an engineer, I appreciate seeing the math and physics. :)


text:
What is this hole in the nose of the A330?
In this question, there is an image of a ladder leading to some hole in the bottom of the nose of an A330. What is that hole for?

Source

Booo!!! -1 for a green circle instead of a red circle!!! (just kiddin')

@FreeMan Was not me though. ;)

It is the avionic compartment access door as described in the Flight Crew Operating Manual!

An inward-opening, manually operated, hinged door gives access to the avionics compartment. This door is in the lower fuselage, forward of the nose landing gear bay. A ladder is stowed inside the compartment adjacent to this door, which may be either be operated from the interior or the exterior.
  This compartment is also accessible from the cockpit, via a floor hatch located behind the Captains' seat. A fixed ladder in the avionics compartment for access from the cockpit.

Here is a drawing from the outside!

And here a view from the cockpit flight deck for an A340!

Like a server room for the airplane. I swear they always make the IT guy work in the worst conditions. ;-)

@ToddWilcox what are you talking about? They even got their own ladder for extra speedy boarding!

Any picture of the inside ?

Your first picture also accidentally answered http://aviation.stackexchange.com/questions/25896/what-is-this-door-above-the-cockpit-of-a-747

That 'hole' gives access to the avionics comparment- forward zone. The comparment has a ground access ladder, which is seen in the image. The deployment of that ladder (which is similar in A330/A340) can be seen in this video.


text:
Why do delta-wings perform so well on paper airplanes?
Based on this question, it seems that delta-shaped wings do not perform well at low airspeeds. But why do paper airplanes, particularly the most well known one, fly well even though the airspeed they fly in is at most 15 knots on average?

Not sure if it really matters as much since I'm not up on my aerodynamics, but also think of scale speed, 15kts for an airplane ~12" long, is pretty fast scaled up. Even to a modest 40', thats 1/40th scale, so essentially its going 600 scale knots. I'm sure the math doesn't work out perfectly like that, but its one thing to consider...

The delta wing has a low aspect ratio, and paper is stiff enough to build a delta paper airplane. If you use a more efficient wing, a sheet of paper is too floppy when you restrict yourself to simple folding (as opposed to cutting and glueing) of the paper.

Your interpretation of the answer to that question is a bit wrong.
It would be an error to claim that delta wings don't perform well at low speed. Delta wings perform exceptionally well at low speeds.
The correct interpretation is delta wings are not fuel efficient. And this is true at low and high speeds.
There are advantages to delta wings at both high and low speeds. The following list are some of the known advantages:

Improved handling at high Mach numbers due to swept leading edge (note: this is the same advantage as swept wings but with the down side of being draggy when performing manoeuvres - swept wing planes retain much more kinetic energy after a turn).
Intrinsically strong structurally (it's a triangle!) thus can be built much lighter.
Retains quite a lot of rudder effectiveness at low speeds and high angles of attack (this is unintuitive because you'd think the wing would block airflow to the rudder but the strong vortex shedding on the leading edge maintains flow around the rudder).
Have very wide CG range compared to conventional aircraft. This is especially useful in bombers.
Have very benign and gentle stall behaviour. This is especially useful for paper airplanes since it can prolong flight at low speeds (paper airplanes simply dip their noses and regain speed instead of simply diving and crashing).

The major disadvantage is that it is draggy. Thus not fuel efficient. Thus in a business world where profit and cost matters delta wings are not attractive.
For supersonic flights the drag is apparently roughly on par with swept wings. Except, as mentioned above, due to the geometry of the wing it can be built lighter thus save fuel - so it sort of cancels out.
For toy planes, where efficiency is not of great concern (fun and "cool" is more important) delta wings are OK.
Still, if you use the same sheet of paper and built a long, narrow, cambered wing like this:

.. you'll find that the long narrow winged plane flies much further than the delta wing paper airplane given the same launch speed.

Delta wings don't perform well at low speeds compared to other low speed wings such as straight ones. The delta wings do have some disadvantages compared to the 'reguar' wings at low speeds. I'm assuming that you're talking about paper aircraft like this:

Image from secretprojects.co.uk
Delta wings have more friction drag, have difficulty in integrating high lift devices and have low AR etc. (as already noted in that question); however, most of these disadvantages are marginal when it comes to the paper aircraft. On the other hand, this kind of design offers some good advantages when you want it to just fly when thrown:

The delta wings still produce lift using a vortex, which means that they stall at much higher angles of attack. At low Reynolds numbers, the max. lift coeffecient is obtained at slightly higher AoA. This helps in their flight.
The centerfold, in addition to helping in holding and throwing, also helps in lateral stability. Also, the flow through that helps in reducing the pressure over the top surface of the paper craft, helping in increasing lift.

They don't perform well on paper airplanes as compared with custom engineered airplane's made from single or ply sheets of paper.
It's just that it's much easier to fold a sheet of paper into a delta wing shaped airplane; hence the reasons most paper airplane shoes have delta wings on them.